1. Field of the Invention
The present invention relates to an electrophotographic printing system. More particularly, it relates to a reversal image development type electrophotographic printing system in which a rotatable means coated with a light sensitive layer and an image developing roller are used in cooperation.
2. Description of the Related Art
Electrophotographic printing (recording) systems are extensively known, and are categorized into two types on the basis of a latent image formation; a positive image development type electrophotographic printing system and a reversal image development type electrophotographic printing system. In the former system, non-image formation areas on the light sensitive layer on a drum charged with approximately several hundreds volts are light-scanned to reduce the voltage there at and toner having an opposite polarity to the light sensitive layer is deposited on other portions on the light sensitive layer retaining a high charge. Conversely, in the latter system, image formation areas on the light sensitive layer are light-scanned to reduce the voltage there at. Toner, having a same polarity as the image formation areas and carrying a charge generated by friction between the carriers and the toner, is deposited on the image formation areas. The present invention essentially pertains to the latter system, i.e., the reversal image development type electrophotographic printing system using a printing medium normally having two components, i.e., carriers and toner.
Due to the principle of reversal image development, in a start-up condition of the reversal image development type electrophotographic printing system, superfluous toner is deposited on the light sensitive layer. Similarly, in a stop condition, the carriers are extracted from the light sensitive layer. These phenomena will be described later in detail with reference to specific embodiments. The above extra toner and carriers are wasted and may shorten a life of a cleaner.
When a mono-component developing medium having resin toner comprising a mixed magnetized powder and resin, and a charge control medium is used, superfluous developing medium extraction also may occur in a start-up condition.
To overcome the above defects, separate drives for the rotatable drum covered with the light sensitive layer and the image developing roller facing the drum and bearing the printing medium thereon, have been disclosed. However, this approach suffers from a disadvantage in that two independent motors must be provided for separately driving the drum and the roller. Thus, a complex control circuit must be provided. Accordingly, this approach greatly increases the cost of the system.